There has been known a technology in which a reducing agent is supplied to a catalyst provided in an exhaust passage of an internal combustion engine to remove NOx, raise the temperature of the catalyst, and/or regenerate a filter. There has also been known a technology of providing an impingement wall on which the reducing agent is to impinge or strike, at a very short distance from the injection port of a reducing agent addition valve (see for example, Patent Document 1). According to this technology, since the reducing agent impinges on the impingement wall before it flows downstream with the stream of the exhaust gas, the atomization of the reducing agent is promoted. In addition, since the injection port is not directly exposed to the stream of the exhaust gas, clogging of the injection port can be prevented.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-89255    Patent Document 2: Japanese Patent Application Laid-Open No. 2006-9608    Patent Document 3: Japanese Patent Application Laid-Open No. 2007-32472    Patent Document 4: Japanese Patent Application Laid-Open No. 2004-532954